OP,
A few years back, I asked Frank White at CLE to turn an 18" full heavy/fluted 1x7.7 SS barrel to get the most out of the 77gr. SMK OTM based BHA MK 262 MOD 1 factory loading or similar 5.56MM NATO pressure 77gr. SMK handload. At that time, he said a rifle length gas system 18" Krieger...